This invention relates to LED lighting systems. More particularly, the present invention relates to LED light output color temperature control and dimming.
Incandescent Light, Luminance, and Dimming
For over a century, incandescent lamps reigned supreme as the most used devices to provide light to humanity. When electrical power (measured in Watts, W) is applied to an incandescent lamp, the incandescent lamp produces light.
The total amount of light (luminance) generated by an incandescent lamp depends upon the amount of electrical power applied to the lamp. That is, increases in the input electrical power to an incandescent lamp causes the lamp to produce greater luminance (brighter light) until a threshold is reached where the incandescent lamp fails due to the high input electrical power, duration of time such power is applied to it, or both.
Likewise, decreases in the input electrical power to an incandescent lamp causes the lamp to produce lesser luminance (dimmed light) until the input electrical power is decreased to a threshold value below which no light is produced by the lamp. Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. The international system of units (SI) unit for luminance is candela per square meter (cd/m2). In the drawings and in this document, luminance is represented by capital letter L.
FIG. 1 is a diagram illustrating, inter alia, relationship between the input power levels (the X-axis) and corresponding output luminance (the first Y-axis) of an incandescent lamp. As illustrated by luminance curve 10 (sold line curve), at a first threshold input power, WTH1, the incandescent lamp begins to produce light at some minimal luminance, LMIN1. The luminance 10 increases as the input power increases until at a second threshold input power, WTH2, the lamp produces light at its maximum luminance, LMAX1. Further increases in the input power beyond the second threshold level, WTH2, would cause the incandescent lamp to fail prematurely and this is not illustrated in the Figures. The luminance curve 10 is a generalized and simplified representation of the relationship between the input power and the output luminance of an incandescent lamp; the curve 10 is used for illustrative purposes only and as an aid to understanding the relationship. For example, the luminance curve 10, as illustrated, may appear to indicate a mostly linear relationship between the input power and the output brightness. However, typically the relationship is closer to logarithmic. Here, the Output Brightness scale (the first Y-axis) may be in logarithmic scale. In any scale, the discussed relationship of increasing power leading to increased luminance output is valid.
Accordingly, the amount of light produced by an incandescent lamp can be controlled by a dimming switch. The dimming switch controls the input power to the incandescent lamp, which, in turn, controls the luminance of the produced light. This dimming effect is useful for many applications including, for example only, ambient mood lighting.
In addition to the dimming effect, changes in the input power level (to the incandescent lamp) change the color temperature of the produced light.
Color Temperature
Color temperature is a characteristic of light that may be defined and understood in a number of different ways. Light is electro-magnetic radiation at a range of frequencies. The perceived color of light depends on the frequency (or wavelength) of the radiation. Most light, especially ambient light such as the light produced by incandescent lamp is a mixture of, or combination of, light have at a range of frequencies (or, differently expressed, at different wavelengths, or “colors”).
Color temperature of light can be understood as the spectral distribution and content of the light. More simply, color temperature is the relative amounts of different “colors” present in the light. Color temperature is measured using a scale having Kelvin (K) units.
For example, a burning candle typically generates light having a wide spectrum of colors; however, in the candle light, the dominant light components have yellow and orange color. Accordingly, overall, candle light is typically characterized as having a color temperature below 1,900 degrees Kelvin. An incandescent lamp typically generates light having a wide spectrum of colors; however, here, overall, incandescent light is typically characterized as having color temperature ranging approximately from 2,500 to 3,500 degrees Kelvin. These two examples are of comparatively low color temperature light having comparatively more yellow to red light components. Such light is generally referred to as being “warm” or “soft” light.
Higher color temperature light has comparatively more white to blue components and is generally referred to as being “cold” or “harsh” light. For example, “white” fluorescent lighting often found at retail spaces and offices is characterized as having color temperature ranging approximately from 3,500 to 4,500 degrees Kelvin. The sunlight at mid summer day has color temperature ranging approximately from 5,500 to 6,000 degrees Kelvin.
Color Temperature Changes During Dimming of Incandescent Lamps
Changes in the input power level to an incandescent lamp not only change the output luminance, but also change the color temperature of the light produced by the incandescent lamp.
FIG. 1 also illustrates relationship between the input power levels (the X-axis) and the color temperature (the second Y-axis) of the light produced by an incandescent lamp at various power levels. As illustrated by color temperature curve 12 (dashed line curve), at relatively higher power levels (and correspondingly higher luminance), the produced light has a comparatively higher color temperature indicated in FIG. 1 as temperature KHIGH. Also illustrated by the color temperature curve 12, as the input power level is decreased (and the luminance reduces as illustrated by curve 10) the color temperature of the produced light also decreases toward a lower color temperature indicated in FIG. 1 as temperature KLOW. That is, the incandescent light has a color temperature range 14 as illustrated. In some applications, lower color temperature light is preferred because the lower color temperature light may be perceived as a warmer, softer light.
For residential ambient lighting applications, the low and the high color temperature values KLOW and KHIGH may range approximately 2,500 to 3,500 degrees Kelvin, respectively. However, the actual values of the color temperature may vary widely outside of these values depending on many factors. The color temperature curve 12 is a generalized and simplified representation of the relationship between the input power and the color temperature of the produced light of an incandescent lamp; the curve 12 is used for illustrative purposes only and as an aid to understanding the relationship.
Incandescent Dimming Effect in Both Luminance and Color Temperature
As discussed above, for incandescent lamps, when input power is dimmed, both the output luminance and the color temperature of the output light are reduced. The result of the dimming is softer, warmer, and more pleasing light. For many lighting applications, this is a desirable characteristic of incandescent lamps.
Incandescent Dimming Effect in Both Luminance and Color Temperature
Even with such desirable operating characteristics, the use of incandescent lamps is being discouraged. In its place, light emitting diodes (LEDs) are being used to provide lighting in many applications. LEDs are much more energy efficient compared to the energy efficiencies of incandescent lamps.
Similar to the incandescent lamps, the luminance of the light produced by LEDs can be varied by varying the input power to the LEDs. However, variations in the input power to the LEDs do not lead to any significant changes of the color temperature of the light produced by an LED. In “white” LEDs that have a blue semiconductor and yellow phosphor, there may reach a point on overdriving the LED that the phosphor would be saturated and only blue light would increase upon further energy input. This would not be good for the longevity of the LED, however. Additionally, there may be a thermal effect that at higher temperatures the spectrum changes slightly, but again this is not good for the LED lifetime.
FIG. 2 is a diagram illustrating, inter alia, relationship between the input power levels (the X-axis) and corresponding output luminance (the first Y-axis) of an LED. As illustrated by luminance curve 20 (sold line curve), at a third threshold input power, WTH3, the LED begins to produce light at some minimal luminance, LMIN2. The luminance 20 increases as the input power increases until at a fourth threshold input power, WTH4, the LED produces light at its maximum rated luminance, LMAX2. The luminance curve 20 is a generalized and simplified representation of the relationship between the input power and the output luminance of an LED; the curve 20 is used for illustrative purposes only and as an aid to understanding the relationship. Accordingly, the amount of light produced by an incandescent lamp can be controlled by a dimming switch. However, changes in the input power level do not result in significant change in the color temperature of the light produced. This is illustrated by color temperature curve 22 (dashed line). Increased input power may cause slight changes in the color temperature of the light from an LED. This may be due to phosphor saturation, thermal changes, or both causing change in the color temperature. This is illustrated as a color temperature range 24. In the Figure, the range 24 is illustrated in exaggerated matter to more clearly indicate the slight range. This color temperature range is not significant and is typically not even perceptible for standard operating range for ambient temperature. In fact, the color temperature range 24 is orders of magnitude lower than the color temperature range 14 (of FIG. 1). Applied power beyond WTH4 is not recommended for the longevity of the device. In the range above WTH4, though there may be phosphor saturation or thermal effects affecting the color temperature, again, this is at the risk of shortening LED life.
That is, dimming of (reducing the input power to) an LED lamp over its recommended operating range results in a dimmer light but not softer or warmer light. In this way, the LED lamp lacks a desired operating characteristic compared to the incandescent lamp. In addition, LEDs present a nonlinear current load to applied electrical voltage, especially when alternating current (AC) power is applied. This may create a high total harmonic distortion (THD). This is an undesirable characteristic of LED lamps.
Accordingly, the need remains for LED based lighting systems having color temperature properties similar to incandescent lighting while maintaining low THD values and high efficiency.